Recently, there is a need for measuring a state such as a temperature, a deformation, or pressure of an object to be measured with a sensor. Among these, an optical fiber FBG sensor is focused because of practical advantages of an optical fiber such as non-inductivity, an explosion proof property, and corrosion resistance. Here, a structure of a conventional FBG sensor is illustrated in FIG. 21. An FBG is an optical device in which a refractive index is periodically changed in a longitudinal direction of a core part 2101 of an optical fiber and a diffraction grating 2103 is formed. The FBG has a property of reflecting only an optical signal of a predetermined wavelength by Bragg reflection. A wavelength of a reflected optical signal (Bragg wavelength) λB is expressed by the following expression (1) using effective refraction neff of an optical fiber core and a width Λ of a refractive index distribution.λB=2neffΛ  [Math 1]
In the conventional FBG sensor, there is a problem that it is not possible to improve an SNR of Bragg reflection even when a light quantity of a light source is increased and that it is not possible to secure an SNR adequate for multipoint measurement in a case of measuring a state of a plurality of positions of an object to be measured.
In PTL 1, a method to improve accuracy of measurement of a wavelength change by making reflection light (signal light) from a measuring FBG sensor and reflection light (reference light) from a reference FBG sensor interfere with each other and by detecting a beat signal corresponding to a wavelength difference between the two and to measure a state (temperature or deformation) of an object to be measured is disclosed.